The invention relates to a valve train of an internal combustion engine, with a camshaft that comprises a carrier shaft and a cam part that is locked in rotation on this carrier shaft and is arranged displaceable in the axial direction and has at least one cam group of directly adjacent cams of different elevations for variable actuation of a gas-exchange valve and a groove-shaped axial connecting link with two connecting-link paths crossing its periphery, and with two actuation pins that can be coupled in the connecting-link paths for displacing the cam part in the direction of the two connecting-link paths.
From DE 101 48 177 A1, a valve train with a cam part that can be displaced between two axial positions is known, whose groove-shaped, axial connecting link is composed merely from external guide walls for specifying the crossing connecting-link paths. For this open construction of the axial connecting link, however, there is considerable risk with respect to the functional safety of the valve train in that the displacement process of the cam part along the currently active connecting-link path is closed completely, i.e., free from incorrect switching, only when the inertia of the moving cam part is sufficiently large for the contact change of the actuation pin required in the crossing region of the connecting-link paths between the external guide walls. This is because, during and after this free-flight phase during the contact change, the cam part must be in the position to move into its other axial position also without positive accelerating forced action of the actuation pin. A prerequisite for sufficiently large inertia of the cam part is a minimum rotational speed of the camshaft that increases with friction between the cam part and the carrier shaft. A displacement of the cam part rotating below this minimum rotational speed can lead to the result that the cam part remains standing “halfway,” namely in the crossing region of the connecting-link paths and a cam follower loading the gas-exchange valve is loaded in an uncontrolled manner by several cams of the cam group and simultaneously with high mechanical loads. In addition, in this case there is no longer the possibility to displace the cam part by the actuation pin at a later time into one of the axial positions, because then the axial allocation between the actuation pin and the external guide walls is no longer set.
For remedying this problem, in DE 10 2008 024 911 A1 it was proposed to provide the cam part with a flexible guide mechanism for the actuation pin. The guide mechanism comprises two guide vanes rotating in opposite directions for formation of inner guide walls of the axial connecting link that can move in the axial direction relative to the rigid, outer guide walls. As for a switch point, here according to the position of the guide vanes, the one connecting-link path is freed for the actuation pin and the other connecting-link path is blocked for the actuation pin. Simultaneously, the inner guide walls also cause an axial forced guidance of the cam part on the actuation pin after passing through the crossing region of the connecting-link paths, so that the displacement process of the cam part is completed without incorrect switching along the currently active connecting-link path.
A valve train according to the class with an axial connecting link having two crossing connecting-link paths and two actuation pins is known from DE 10 2007 051 739 A1. The interaction of the groove-shaped axial connecting link with the actuation pins coupled selectively therein allows the presentation of a cam group with three cams, i.e., a three-stage variable valve train. As in the first-cited publication, however, the axial connecting link has only outer guide walls, so that there is also a correspondingly high risk for incorrect switching of the cam part also for this valve train.
For complete clarification it should be noted that the terms before, in, or after the crossing region always relate to the starting position of the actuation pins relative to the axial connecting link rotating with a fixed rotational direction on the cam part.